In the context of treating rapid ventricular tachycardias, it is useful to be able to determine whether the rapid rate is accompanied by a substantial drop in cardiac output. If so, a cardioversion or defibrillation level shock may be required. If not, pacing therapies may be usefully applied, at least initially. As high level shocks are painful, patients generally prefer to avoid their unnecessary application.
To this end, it has been proposed to incorporate hemodynamic sensors of various sorts into automatic implantable cardioverter defibrillators (ICDs), to allow the devices to deliver the most appropriate therapies. One approach has been to measure intravascular blood flow as an indicator of cardiac output, i.e. low flow implies low output. Such devices are generally disclosed in U.S. Pat. No. 5,782,879 issued to Rosborough, et al., U.S. Pat. No. 5,520,190 issued to Benedict, et al. and U.S. Pat. No. 5,409,009, issued to Olson, all of which are incorporated herein by reference in their entireties.
While not employed in the above-cited patents, it is also known that cardiac output and blood flow can be measured by determining the result of heat by means of an intravascular heater. In general, the more blood flow, the greater the dissipation of heat from the heater. Exemplary devices are disclosed in U.S. Pat. No. 6,387,052, issued to Quinn, et al and US Patent Application Publication No. US2003/0225336 A1, by Callister, et al., both of which are also incorporated herein by reference in their entireties.